1. Field of the Invention
The present invention relates to a polymer electrolyte and a lithium polymer battery using the same.
2. Description of the Prior Art
Lithium secondary batteries characterized by their high voltage and high energy density are highly promising as the new-type secondary batteries of the next generation, and therefore, vigorous researches and developments directed to their improvements are under progress. In an attempt to improve their ionic conductivity and decomposition voltage, electrolytes for the lithium secondary batteries have also been briskly developed.
An organic electrolyte solution (wherein cations and/or anions are dissolved in an organic solvent) is employed in most of the lithium secondary batteries including the lithium ion secondary battery. In practical batteries, a separator in a shape of porous membrane made of polypropylene or the like is impregnated with the organic electrolyte solution, thereby to secure ion-conductive channels across the positive electrode and the negative electrode.
The conventional lithium secondary batteries frequently have the problems of internal short-circuiting due to formation of lithium dendrites, and it is difficult to control the formation, particularly in a system using the organic electrolyte solution. The organic electrolyte solution is a fluid and thus cannot suppress the growth of dendrites.
If a separator is used, a current flow passing across the positive electrode and the negative electrode concentrates on the microporous portions of the separator which are offered for limited ion-conductive channels. As a result, the growth of lithium dendrites is promoted intensively on the microporous portions of the separator.
In order to break through these situations, a battery system utilizing a solid polymer electrolyte has been proposed and is now under development. Such polymer electrolyte is an ionic conductor which has metal salts dissolved homogeneously. This functions as a solid electrolyte in a separator-free battery, and thus can suppress the formation and the growth of lithium dendrites effectively because the current flows across both electrodes uniformly over the entire surface of the polymer electrolyte.
The ionic conductivity of the polymer electrolyte is however about 10.sup.-5 S/cm at room temperature which is lower than that of any organic electrolyte solution by 100 times or more. The low ionic conductivity is a main factor responsible for disturbing the practical introduction of the polymer electrolyte into the commercial secondary batteries, and thus the research and development are now directed to the improvement of this low conductivity.
Under these circumstances, in an attempt to secure an ionic conductivity of the same degree as that of the organic electrolyte solution, the development of a gelled polymer electrolyte has been promoted. The gelled polymer electrolyte is prepared by impregnating a polymer matrix with an electrolyte solution wherein the ion conduction is mainly performed through the phase of the electrolyte solution.
Also the gelled polymer electrolyte has another advantage, that is, a higher physical pressure effect than that of the organic electrolyte solution on the portions where the current flows, and thus the gelled polymer electrolyte has a higher suppressive effect on the growth of the lithium dendrite.
The gelled polymer electrolytes are roughly classified into two groups according to the species of the employed polymer matrices: One is a gel of three-dimensionally cross-linked polymer formed by mixing a monomer having an acrylate terminal group with an organic electrolyte solution and polymerizing the mixture to be cured by means of an ultraviolet rays or an electron beam radiation. An example is disclosed in, for instance, U.S. Pat. No. 4,830,939, which utilizes the cured three-dimensionally cross-linked polymer as the gel. The other is a gel formed by impregnating a one-dimensional straight chain polymer such as polyethylene oxide or the like with the organic electrolyte solution.
Either of the gels demonstrates an ionic conductivity of the same degree as that of the organic electrolyte solution, The three-dimensionally cross-linked polymer gel however suffers from a disadvantage that its cross-linked point is liable to be oxidized at high voltage over 4 V and thus it has a low decomposition voltage. Therefore, the one-dimensional straight chain polymer gel is preferable as the electrolyte for the polymer battery of 4 V class which uses a positive electrode active material of LiCoO.sub.2, LiNiO.sub.2 or the like.
The gel obtained by impregnating a film of the one-dimensional straight chain polymer with the organic electrolyte solution demonstrates an ionic conductivity of the same degree as that of the organic electrolyte solution. The gel however cannot maintain the shape of the film because the polyethylene oxide dissolves in the electrolyte solution and flows, thereby to lose its function as a solid and decrease its mechanical strength.
By contrast, when a film made of polyvinylidene fluoride, which is one-dimensional straight chain polymer similar to the polyethylene oxide, is immersed in the organic electrolyte solution, dissolution of the polymer and its flowing are not occurred. The obtained film is swollen only slightly but functions as a gel having a self-supporting property. The ionic conductivity of the film cannot however be high enough, and the gel of polyvinylidene fluoride as disclosed in Japanese Laid-Open Patent Publication Sho 61-23947 has a low conductivity of about 10.sup.-5 S/cm at best.
Under these circumstances, for realizing practical use of the one-dimensional straight chain polymer as the polymer matrix, it is necessary to satisfy both requirements of maintaining the shape as a solid and securing an acceptable ionic conductivity at the same time. Either of the functions is deteriorated in the conventional electrolytes and thus development of such a polymer material that satisfies both of the above-mentioned two requirements at the same time has been eagerly desired.